The present invention generally relates to an apparatus for holding a wafer in a wet etch tank and a method for using, more particularly, relates to an apparatus for holding a wafer in a wet etch tank that does not require mechanical clamping for etching the wafer backside and a method for using the apparatus.
Miniaturization of motors, actuators and similar machine parts is receiving increasing attention because of the new uses of these devices made possible because of their small size. Additionally, these devices can be manufactured in large quantities at low piece-part cost. Current designs of miniaturized machine parts can be categorized according to size or scale. Macroscopic machine parts have a length in the range of approximately 1 to 10 inches, and while microscopic machine parts, sometimes referred to as MEMS (Micro-Electro-Mechanical-Systems) have a length in the range of 0.01 to 1 inch.
In any event, existing miniaturized actuators and motors of both macroscopic or microscopic size are essentially replicas of larger motors, and thus include such component parts as windings, stators, gears, transmission links, etc. These miniaturized parts must be assembled with high precision in order to produce an operable device providing the desired function, e.g. movement of an electrically activated component that then mechanically engages other parts to induce motion. Depending upon the engagement configuration, this motion may be linear in any of several axes, rotary, circular, etc. Because of the number of complex parts that must be assembled with a high degree of precision, the yields of parts meeting target specifications and performance are relatively low using current manufacturing processes. These low yields in turn increase the cost of the parts. Accordingly, it would be desirable to provide a new form of actuator and related method for inducing movement of an object on a microscopic or macroscopic scale which eliminates the problems mentioned above.
The MEMS technology has recently been extended to the semiconductor fabrication industry. In the present state of the art, a semiconductor device is normally formed in a planar structure and therefore the process for fabricating the semiconductor device is generally a planar process. For instance, layers of different materials, i.e. such as insulating materials and metallic conducting materials, are deposited on top of one another and then features of the device are etched through the various layers. The planar fabrication process, while adequate in fabricating most semiconductor elements and devices, is not suitable for fabricating certain devices that are 3-dimensional in nature. For instance, a 3-D solenoid, i.e. or a 3-D inductor coil, must be fabricated by stacking a large number of layers from the bottom to the top and therefore, requires a large number of photomasks to complete the task. When CMOS technology is used in forming such 3-D solenoid, at least four other steps utilizing photomasks must be incorporated in order to complete the fabrication process. Moreover, the precise alignment between the layers is necessary in order to avoid a variety of processing difficulties occurring at the interfaces.
Another integration between the CMOS technology and the MEMS technology is the fabrication of a membrane device for a sensor utilized in an integrated circuit. The formation of the membrane requires the deposition of a silicon nitride film as a passivation layer on the backside of a wafer, the opening of holes by lithography for the membrane, and then wet etching in a caustic solution the wafer backside to form a thin film membrane having a thickness such as 25 xcexcm. For instance, when a 25 xcexcm thickness membrane is to be formed in a 4xe2x80x3 wafer of 525 xcexcm thickness, the wafer backside must be wet etched in a caustic solution of KOH at 30% concentration and 85xc2x0 C. for a length of time as long as 6 hours. The etch rate of silicon in the KOH etchant is about 1.33 xcexcm per minute.
Conventionally, a wafer is held in an apparatus shown in FIGS. 1A and 1B during the wet etch process. Due to the extended length of time, i.e. 6 hours, which is required to etch away the bulk of silicon to leave a thin film membrane, the conventional apparatus 10 utilizing mechanical clamping presents numerous problems. For instance, as shown in FIG. 1A, the clamping screws 12 cause localized stress in the wafer since only six clamping screws are normally used. The clamping stress imposed on the wafer frequently causes cracking or breakage of the wafer, even though O-ring type gaskets 14 are normally used. While the O-ring type gaskets 14 are utilized to alleviate stress concentration, and more importantly, to prevent leakage of the etchant solution to the front side 16 of the wafer 20, the seal provided by the O-ring gaskets 14 is not always effective considering the length of time that the wafer 20 must be immersed in the wet etchant.
FIG. 1A further illustrates six apertures 16 evenly distributed in the upper clamp ring 18 which engages the lower clamp ring 22 such that the backside 24 of the wafer 20 can be exposed to the wet etchant. The potential leakage of the etchant solution to attack the front side 16 of the wafer 20 and the potential breakage of the wafer lead to low yield of the wet etch process.
It is therefore an object of the present invention to provide a process integration for the CMOS process and the MEMS process for etching wafer backside without the drawbacks or shortcomings of the conventional method.
It is another object of the present invention to provide an apparatus for mounting a wafer and etching a wafer backside when the apparatus is immersed in an etchant solution.
It is a further object of the present invention to provide an apparatus for mounting a wafer and etching a wafer backside that does not require mechanical clamping and thus localized clamping stress is avoided.
It is another further object of the present invention to provide an apparatus for mounting a wafer and etching a wafer backside by utilizing a suction force for holding the wafer in the apparatus.
It is still another object of the present invention to provide an apparatus for mounting a wafer and etching a wafer backside by flowing air or inert gas into an outer chamber cavity and withdrawing air from an inner chamber cavity for holding down the wafer.
It is yet another object of the present invention to provide a method for mounting a wafer and etching a wafer backside without mechanical clamping and without wafer breakage problems.
In accordance with the present invention, an apparatus and a method for mounting a wafer and etching a wafer backside are provided.
In a preferred embodiment, an apparatus for mounting a wafer and etching a wafer backside is provided which includes a first circular disc that has a first sidewall integrally formed along a peripheral edge of the disc extending upwardly to a first height; a gas inlet in the first circular disc or in the first sidewall for flowing a gas through the gas inlet into a first cavity formed by the first circular disc, the first sidewall, a second circular disc and a second sidewall; a second circular disc that has a second sidewall integrally formed along a peripheral edge of the disc extending upwardly to a second height, the second circular disc has a diameter smaller than a diameter of the first circular disc by not more than 5 mm, the second sidewall has a height smaller than a height of the first sidewall by at least xc2xd of a thickness of the wafer; a third circular disc that has a third sidewall integrally formed along a peripheral edge of the disc extending upwardly to a third height, the third circular disc has a diameter smaller than a diameter of the second circular disc by at least 5 mm, the third sidewall has a top substantially leveled to a top of the second sidewall, the third circular disc is affixed to and supported by the second circular disc; a gas outlet in the second circular disc for allowing the withdrawal of a gas from a second cavity formed by the second circular disc, the second sidewall, the third circular disc, the third sidewall and the wafer positioned on top and supported by the second and third sidewalls; and sealing means positioned on top of the second and third sidewalls for providing a substantially vacuum-tight chamber in the second cavity when the wafer is positioned on top of the second and third sidewalls.
In the apparatus for mounting a wafer and etching a wafer backside, the first, second and third circular discs and the first, second and third sidewalls may be fabricated of stainless steel or teflon. The second circular disc may have a diameter smaller than a diameter of the first circular disc by an amount sufficient to stop an etchant solution from entering the first cavity when the apparatus is immersed in the etchant solution and when a gas is flown into the gas inlet. The sealing means may be elastomeric O-rings, or may be elastomeric gaskets. The first and second sidewalls may have a top edge equipped with a flat surface sufficiently large to engage the sealing means. The top edges of the first and second sidewalls are substantially in the same horizontal plane. The apparatus may further include fixing means for fixing the position of the second circular disc to the first circular disc, or an aspirator means in fluid communication with the gas outlet for withdrawing gas from the second cavity, or a gas supply means in fluid communication with the gas inlet for flowing a gas into the first cavity.
The present invention is further directed to a method for mounting a wafer and etching the wafer backside which can be carried out by the operating steps of first providing an apparatus that includes a first circular disc that has a first sidewall integrally formed along a peripheral edge of the disc extending upwardly to a first height; a gas inlet in the first circular disc or the first sidewall for flowing a gas through the gas inlet into a first cavity formed by the first circular disc, the first sidewall, a second circular disc and a second sidewall; a second circular disc that has a second sidewall integrally formed along a peripheral edge of the disc extending upwardly to a second height, the second circular disc has a diameter smaller than a diameter of the first circular disc by not more than 5 mm, the second sidewall has a height smaller than a height of the first sidewall by at least xc2xd of a thickness of the wafer; a third circular disc that has a third sidewall integrally formed along a peripheral edge of the disc extending upwardly to a third height, the third circular disc has a diameter smaller than a diameter of the second circular disc by at least 5 mm, the third sidewall has a top substantially leveled to a top of the second sidewall, the third circular disc being affixed to and supported by the second circular disc; a gas outlet in the second circular disc for allowing the withdrawal of air from a second cavity formed by the second circular disc, the second sidewall, the third circular disc, the third sidewall and the wafer positioned on top and supported by the second and third sidewalls; sealing means positioned on top of the second and third sidewalls for providing a substantially vacuum-tight chamber in the second cavity when the wafer is positioned on top of the second and third sidewalls; positioning the wafer on top of the second and third sidewalls; withdrawing air from the second cavity by an evacuation means; flowing air or inert gas into the first cavity forming a barrier to stop the etchant solution from entering the first cavity; immersing the apparatus in an etchant solution and etching a backside of the wafer.
The method for mounting a wafer and etching the wafer backside may further include the step of withdrawing air from the second cavity by an aspirator, or the step of flowing nitrogen into the first cavity forming a barrier to stop the etchant solution from entering the first cavity. The method may further include the step of providing the etchant solution in an etchant tank, or providing the etchant solution in an acid solution in a tank. The method may further include the step of providing the sealing means in an elastomeric gasket, or in an elastomeric O-ring. The method may further include the step of flowing air or inert gas into the first cavity at a pressure higher than a pressure for withdrawing air from the second cavity by the evacuation means, or the step of withdrawing air from the second cavity at a pressure higher than a liquid pressure of the etchant solution, or the step of supplying the wet etchant in KOH solution.